#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main() {
    int score = 0;
    scanf("%d", &score);
    if (score <= 100 && score >= 0)
    {
        switch (score / 10)
        {
        case 10:
        case 9:
            printf("A\n");
            break;
        case 8:
            printf("B\n");
            break;
        case 7:
            printf("C\n");
            break;
        case 6:
            printf("D\n");
            break;
        default:
            printf("E\n");
            break;
        }
    }
    else
        printf("F\n");
    return 0;
}
